Silicates · Sourced PROFILE
Topaz
Al₂SiO₄(F,OH)₂
A hard aluminium silicate mineral valued both as a gemstone and geological indicator.
AT A GLANCE
- Formula
- Al₂SiO₄(F,OH)₂
- Crystal system
- Orthorhombic
- Space group
- Pbnm
- Mohs hardness
- 8
- Density
- 3.49–3.57 g/cm³
- Optical character
- Biaxial (+)
CRYSTALLOGRAPHY
Crystal form & structure
Prismatic crystals, commonly with many faces and vertical striations; also granular or massive.
HAND-SAMPLE PROPERTIES
Physical properties
- Cleavage
- Perfect basal cleavage on {001}
- Fracture
- Subconchoidal to uneven
- Tenacity
- Brittle
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Observed colours
- Colourless, yellow, brown, blue, pink, red, orange, green, or violet
LIGHT & OPTICS
Optical properties
- Optical class
- Biaxial (+)
- Refractive index
- nα about 1.606–1.634; nβ 1.609–1.637; nγ 1.616–1.644
- Fluorescence & electrical behaviour
- Usually inert to weak; some material fluoresces under long- or short-wave UV.
GEOLOGICAL SETTING
Formation
Typically forms in fluorine-rich granitic rocks, pegmatites, and hydrothermal cavities.
MINERAL COMPANY
Common associations
Quartz, fluorite, cassiterite, tourmaline, beryl, feldspars, mica, and wolframite
FIELD NOTES
How to identify it
Hardness 8 is high, but perfect basal cleavage makes topaz vulnerable to a sharp blow. Distinguish it from quartz by cleavage, higher density, and biaxial optics.
NAME & HISTORY
Where the name comes from
Traditionally linked either to Greek Topazios, an island name, or Sanskrit tapas, meaning fire.
SCIENTIFIC SOURCE
Handbook of Mineralogy — Topaz
External reference; factual profile paraphrased and attributed. Copyright remains with the publisher.
PROVENANCE
Documented occurrences
Rangkul' pegmatite field, Eastern Pamir · Topaz occurs with gem tourmaline in miarolitic granite pegmatites of the Rangkul' field, near the Turakuloma (Kukurt) ruby district. The record stays at regional precision.
Sourced · Region precisionMindat — Rangkul' pegmatite field, Rangkul' Highlands, Gorno-Badakhshan